Loans - Troubled Debt Restructurings (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	$ 60,123	$ 76,185
Balance at period end	58,898	34,099
Commercial Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	4,026	510
Balance at period end	3,875	505
Commercial and industrial, Energy [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	56,097	73,977
Balance at period end	55,023	31,918
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	1,455
Balance at period end	0	1,455
Construction Loans [Member]
Financing Receivable, Modifications [Line Items]
Balance at time of restructuring	0	243
Balance at period end	$ 0	$ 221